<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>

	.father{
		width: 500px;
		height: 500px;
		background: red;
		margin-top:50px;
		margin-left:50px;
		position: absolute;
	}
	.son1{
		width: 300px;
		height: 300px;
		background: green;
		/*position: relative;*/
	}
	.son2{
		width: 100px;
		height: 100px;
		background: blue;
		position: absolute;
		top:20px;
		left:20px;
	}
	</style>
</head>
<body>
	<div class="father">
		<div class="son1">
			<div class="son2"></div>
		</div>
	</div>
</body>
</html>